Evaporative emissions control systems in vehicles may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations, and then purge the stored vapors during a subsequent engine operation. In an effort to meet stringent federal emissions regulations, evaporative emissions control systems may be intermittently diagnosed for the presence of vapor leaks that could release fuel vapors to the atmosphere.
Hybrid vehicles and other vehicles with inherently low manifold vacuum may perform leak tests for vapor leaks while the vehicle is turned off. In some examples, the leak test is based on natural pressure or vacuum that occurs in the fuel tank due to fuel heating or cooling. An example approach to infer degradation of the evaporative emissions control system based on natural vacuum following engine-off is shown by Dawson in U.S. Pat. No. 6,073,487. Herein, a leak test includes sealing the fuel system and the evaporative emissions control system following the engine shut-off event and monitoring changes in pressure within the evaporative emissions control system. As such, cooling of fuel within the fuel system generates a vacuum, and integrity of the evaporative emissions control system is determined based on the vacuum attaining a threshold. If the vacuum attains the threshold, a vacuum switch is closed and a controller determines that the system is robust. On the other hand, if vapor leaks are present, vacuum in the evaporative emissions control system does not reach the threshold, and the vacuum switch may not close. In response to the vacuum switch remaining open, the controller determines that vapor leaks are present in the evaporative emissions control system.
The inventors herein though have recognized a potential issue with such approaches. As an example, the vacuum switch may be a mechanical switch (e.g., including a diaphragm) that is designed to close when vacuum in the evaporative emissions control system reaches the threshold. In other words, the vapor leak test may rely on a threshold that is a single constant value based on the design of the vacuum switch. However, in regions that experience low diurnal temperature variations (e.g., wherein temperature variation over a given day is lower), the fuel may not experience significant cooling to generate sufficient vacuum. Accordingly, the vapor leak test may indicate degradation despite the evaporative emissions control system being robust (e.g., a false fail). On the other hand, during windy conditions or when a thunderstorm is occurring, vacuum in the fuel system can attain the threshold easily and may result in the evaporative emissions control system being declared robust even though vapor leaks may be present (e.g., a false pass). Thus, leak tests using the mechanical vacuum switch based on the single constant value threshold may produce unreliable results. Further, if vapor leaks are not diagnosed accurately, the vehicle may either not be emissions compliant (due to the false pass) or may lead to unnecessary expenses to the vehicle operator due to potentially needless vehicle service (due to false fails).
The inventors herein have recognized the above issue and have developed systems and methods to at least partially address them. One example approach includes a method comprising following a vehicle-off event, waking an electronic controller to indicate an absence of a leak in an evaporative emissions system responsive to vacuum in the evaporative emissions system attaining a vacuum threshold, the vacuum threshold determined at the vehicle-off event by the electronic controller and the vacuum threshold based on ambient conditions and fuel conditions. In this way, the vacuum threshold may be a variable threshold. A technical effect of using a threshold that varies based on ambient and fuel conditions is that the leak test may be more robust.
In another example, a method may comprise controlling fuel delivered to an engine by a controller, before turning off the controller after shutdown of the engine, estimating pressure an evaporative emissions system would achieve after the shutdown based on ambient conditions and fuel conditions, and determining whether a vapor leak exists in the evaporative emissions system after the controller turnoff in response to a comparison of actual pressure of the evaporative emissions system to the estimated pressure. In this way, the vapor leak test may occur with lower power consumption during engine shutdown.
In yet another example, a system for a vehicle may comprise a fuel system including a fuel tank, an evaporative emissions system coupled to the fuel system, a pressure sensor coupled to the evaporative emissions system and the fuel system, the pressure sensor measuring a pressure in the evaporative emissions system and fuel system, a powertrain control module comprising a wake input, and a comparator circuit coupled to the wake input, the powertrain control module configured with instructions stored in non-transitory memory that when executed cause the powertrain control module to, in response to a vehicle-off event, estimate a vacuum threshold based on each of ambient conditions and fuel conditions, sleep following estimating the vacuum threshold while maintaining the comparator circuit awake, and responsive to vacuum in the evaporative emissions system reaching the vacuum threshold, wake up and indicate a robust evaporative emissions system, and sleep following the indication. In this way, the vacuum threshold may be different for each leak test. Further, by using the pressure sensor, instead of a mechanical switch, different vacuum thresholds may be sensed. Further still, the leak test may provide more accurate results as existing ambient conditions and fuel conditions are taken into account when determining the vacuum threshold. Overall, performance of the evaporative emissions control system may be enhanced and the vehicle may be emissions compliant.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.